Bug 1521907 part 5. Start using CheckedUnwrapStatic/Dynamic in XPConnect. r=peterv
I am not a huge fan of the UnwrapReflectorToISupports setup here. Maybe we
should introduce two differently-named methods that make it somewhat clear what
the limitations of not taking a JSContext are? I couldn't think of sane
naming...
Differential Revision: https://phabricator.services.mozilla.com/D17885

Bug 1521907 part 3. Start using CheckedUnwrapStatic/Dynamic in bindings. r=peterv
The basic idea for the changes around UnwrapObjectInternal and its callers
(UnwrapObject, UNWRAP_OBJECT, etc) is to add a parameter to the guts of the
object-unwrapping code in bindings which can be either a JSContext* or nullptr
(statically typed). Then we test which type it is and do either a
CheckedUnwrapDynamic or CheckedUnwrapStatic. Since the type is known at
compile time, there is no actual runtime check; the compiler just emits a call
to the right thing directly (verified by examining the assembly output on
Linux).
The rest of the changes are mostly propagating through that template parameter,
adding static asserts to make sure people don't accidentally pass nullptr while
trying to unwrap to a type that might be a WindowProxy or Location, etc.
There are also some changes to places that were calling CheckedUnwrap directly
to use either the static or dynamic version, as needed.
Differential Revision: https://phabricator.services.mozilla.com/D17883

Bug 1521907 part 1. Add a version of CheckedUnwrap that can do a dynamic security check. r=jandem,sfink
We're going to need this because we will have multiple Realms in the same
compartment which want different CheckedUnwrap behavior in some cases. So we
need to be able to check which Realm we're in.
Differential Revision: https://phabricator.services.mozilla.com/D17881